1. Field of the Invention
The invention relates to a shaft fixed magnetic disk drive apparatus for rotating magnetic disks while having the magnetic disks attached thereto.
2. Related Art
A magnetic disk drive apparatus is designed to write and read data signals with respect to magnetic disks by attaching the magnetic disks to the outer circumferential surface of a rotatably supported hub and by rotating the hub to rotate the magnetic disks. Such magnetic disk drive apparatus can be classified into two types in terms of how the hub is rotatably supported: a shaft rotated type and a shaft fixed type. The shaft rotated type is characterized as rotatably supporting the shaft on the inner circumferential side of a bearing housing through bearings, and rotating the hub integrally with the shaft. The shaft fixed type is characterized as mounting bearings on the outer circumferential side of the fixed shaft, and rotatably supporting the hub by engaging the center hole of the hub with the outer circumferential side of the bearings. In either type, in order to give preloads to the bearings, a gap is provided between the inner ring of at least one bearing and the shaft before the preloads are given so that the inner ring of the bearing is slidable relative to the shaft.
According to the shaft rotated disk drive apparatus, the disk clamp for clamping the magnetic disks attached to the outer circumferential surface of the hub can be mounted by inserting only one screw into a tap hole formed at the center of the distal end portion of the rotating shaft, which is an advantage. However, since oscillations of the hub and hence oscillations of the magnetic disks are dependent on the gap between the inner ring of the bearing and the rotating shaft, if the oscillations of the magnetic disks are relatively large, it is difficult to control, e.g., oscillations in the axial direction to 25 .mu.m or less. Further, in order to minimize the oscillations of the magnetic disks, lacing is provided on the outer circumferential surface of the hub with the magnetic disk drive apparatus already assembled. This entails a large number of assembling and metal working steps.
On the other hand, according to the shaft fixed magnetic disk drive apparatus, oscillations in both axial and radial directions of the hub and the magnetic disks are dependent on the gap between the outer ring of the bearing and the inner diameter of the hub. Since the hub is press-fitted into the outer ring of the bearing, there is no gap between the outer ring of the bearing and the inner diameter of the hub. Therefore, oscillations of the magnetic disks can be controlled within 5 .mu.m without lacing the hub with the magnetic disk drive apparatus already assembled, thereby allowing the highly accurate magnetic disk drive apparatus to be obtained at a relatively low cost. A conventional shaft fixed magnetic disk drive apparatus will hereunder be described with reference to the drawings.
In FIGS. 9 and 10, a frame portion 10 has a center fixed shaft 12 so as to be integral therewith. The center fixed shaft 12 is cylindrical and extends upward from the central portion of the frame portion 10. On upper and lower portions of the outer circumference of the center fixed shaft 12 are inner rings 16, 22 of two ball bearings 14, 20. Between the inner ring 16 of the upper bearing 14 and the center fixed shaft 12 is a small gap so that the inner ring 16 is slidable relative to the center fixed shaft 12 until a preload is applied thereto. Outer rings 18, 24 of the respective bearings 14, 20 are press-fitted into a center hole 25 of a hub 26. The hub 26 has a jetty 82 on the inner circumferential edge of a distal end (the top end as viewed in FIGS. 9 and 10) thereof, the jetty 82 being circular as viewed in the axial direction. A cap 78 for closing the center hole 25 of the hub 26 is accommodated in the jetty 82. Axially extending tap holes 80 are provided at several positions (usually 4) on an outer circumferential side of the distal end portion of the hub 26, the outer circumferential side being further out of the jetty 82. A step 84 is formed at a lower portion of the hub 26 by increasing the outer diameter of the hub, and from the step 84 extends a circumferential wall 86 integrally therewith.
The inner rings 16, 22 and outer rings 18, 24 of the respective bearings 14, 20 contact the outer circumferential surface of the center fixed shaft 12 and the inner circumferential surface of the hub 26, respectively, under the condition that the inner rings 16, 22 of the respective bearings 14, 20 are given preloads in directions indicated by arrow (a) by biasing the inner ring 16 of the bearing 14 upward in the axial direction. This type of preload is called a static preload. Seals 51, 52 are arranged on both ends of each of the respective bearings 14, 20 so as to prevent splashes of a lubricant and the like from being scattered outside.
As shown in FIG. 10, an appropriate number of magnetic disks 40 (2 disks in the example shown in FIG. 10) are attached to the outer circumferential surface of the hub 26. More specifically, a magnetic disk 40, a spacer 42, and a magnetic disk 40 are set down along the outer circumferential surface of the hub 26 in this order, and these magnetic disks 40 and the spacer 42 are stopped by the step 84 of the hub 26. A disk clamp 90 made of a spring member is fitted on the outer side of the jetty 82 of the hub 26 and is mounted with setscrews 92 being screwed into the appropriate number of tap holes 80. The disk clamp 90 has a plurality of spring leg portions 91 extending radially from a ringlike base portion thereof. The respective magnetic disks 40 and the spacer 42 are fixedly secured to the hub 26 integrally with one another by these spring leg portions 91 biasing the upper magnetic disk 40 downward with resilient forces thereof.
A ringlike drive magnet 46 is fixedly secured to the inner circumferential surface of the circumferential wall 86 of the hub 26 through a yoke 88. The outer diameter of the frame portion 10 is set to a value larger than that of the center fixed shaft 12. A stator core 48 is secured to the outer circumferential side of the frame portion 10 by engagement. The stator core 48 has an appropriate number of salient poles in the radial direction thereof, each salient pole having a drive coil 50 wound. The outer circumferential surface of the stator core 48, i.e., the tip of each salient pole confronts the inner circumferential surface of the drive magnet 46 while interposing a predetermined gap.
When the drive coils 50 are energized alternately in accordance with the rotational position of the drive magnet 46, the drive magnet 46 as well as the hub 26 and the magnetic disks 40 integrated with the drive magnet 46 are rotated. Since the bearing 14 is exposed from the distal end portion of the hub 26 through the center hole 25 in the thus constructed shaft fixed magnetic disk drive apparatus, the cap 78 is used to cover the center hole 25 to prevent splashes of the lubricant and the like from adhering to the surfaces of the magnetic disks 40.
It is true that the shaft fixed magnetic disk drive apparatus can provide the advantage that oscillations of the hub 26 and the magnetic disks 40 can be minimized and that the highly accurate magnetic disk drive apparatus can be obtained at a relatively low cost as described above. However, in the shaft fixed magnetic disk drive apparatus, as shown in FIGS. 9 and 10, the disk clamp 90 must be screwed at a plurality of positions on the circumferential edge of the hub 26. This brings about variations in the depth of the tap holes 80, variations in the tightening torque of the screws 92, and the like, which then deteriorates the rotational balance between the hub 26 and the magnetic disks 40, thus causing, in some cases, precessions and oscillations during rotation.